1. Field of the Invention
The present invention relates to a magnetic disk apparatus in a computer system, more particularly, to a track access control system for a magnetic disk apparatus wherein a magnetic disk is provided with data tracks and servo tracks and the amount of offset obtained from the servo tracks is used as the amount of correction for accessing the data tracks.
Further, the present invention relates to a magnetic disk apparatus which uses a positioning system based on an open loop system typically used for stepping motors or an open loop system combining a DC torque motor and encoder, and which uses a so-called comparative track servo system based on a semiclosed loop system wherein the servo position information recorded on the data surface is used as feedback control information.
2. Description of the Related Art
The track pitch on the disks of rotary disk memory devices such as magnetic disk apparatuses, etc., has recently become much smaller due to the demands for a higher density and larger capacity data storage. In such disk apparatuses, the small track pitch means that deviations in the track position caused by an imprecise mounting of the magnetic head or elongation or contraction of the disk etc., due to temperature rises can no longer be ignored, and thus a need has arisen for the detection of the deviations in position and correction of the same.
Therefore, magnetic disks are now provided with not only data tracks, but also servo tracks, on part of which is written servo positioning information. The magnetic head is positioned at the servo tracks and the content of the servo tracks read out by the magnetic head and deviations in position of the magnetic head vis-a-vis the servo tracks are detected by the read output. This is used as the amount of offset and as the amount of correction in the later accessing by the head of the data tracks.
On the other hand, there is a need for updating the amount of offset, since the deviations in position change along with temperature.
One method for updating the offset is to access the servo tracks periodically, measure the amount of offset, and then update the same. The offset update, however, is performed asynchronously with the host controller, and if the offset update operation is being performed while the host controller is giving a seek command, this command could not be accepted. Therefore, the host controller must monitor offset update operations of the disk apparatus while issuing commands.
Various track access control system for disk apparatuses able to perform the above-mentioned offset update without obstructing the issuance of commands by the host controller have been proposed.
On the other hand, various positioning systems for magnetic disk apparatuses have been proposed, among which are known (a) a semiclosed loop system wherein use is made of a stepping motor, open loop control is adopted, and the positional information recorded on the data surface of the magnetic disk is fed back, and, (b) another semiclosed loop system wherein a DC torque motor and encoder are combined, open loop control is adopted, and, in the same way as above, positional data is fed back.
Generally, there are index servo systems which use indexes as positional information and comparative track servo systems which use the offset of the servo track.
The comparative track servo system operates on the basic principle of seeking the servo tracks to be compared near the target cylinder, synchronously with a seek command from the host controller or disk controller, and reading the servo information written there, using the read servo information for fine adjustment, and simultaneously seeking the target cylinder and positioning there. Here the amount of offset is updated just before the positioning to the target cylinder. But in this system, if a seek command is not issued by the host controller for a long time, servo information is not read out and thus the amount of offset is not updated. Therefore, if temperature changes occurred in the mechanical components during that time, the problem of off-tracking due to heat expansion would still exist.
One method of preventing this offset was disclosed by the assignee in JPA62-162546, entitled "Track Access Control System for Disk Apparatus". This system made use of the fact that a track seek command is issued when the host controller encounters a read error. That is, a timer is provided in the magnetic disk apparatus side and if no seek command arrives after a set period of time, the timer runs out, the amount of offset is made invalid, and a read error is compulsorily generated. The host controller then issues a retry seek command in response to the read error, and the magnetic disk apparatus then updates the amount of offset in accordance with the retry seek command.
It is important to note that the magnetic disk apparatus performing the above processing has great value as an OEM product and can be connected to various types of host controllers; in other words, it has an interface which enables it to be used in various systems. Also, when performing the above-mentioned processing, the existing processing content of the host controller is not changed.
The above method, i.e., the method which compulsorily causes a generation of a read error, cannot be used as is for a host controller which does not issue a retry seek command even if a read error is generated. In general, as mentioned earlier, the host controller has a retry seek command issuing function, but sometimes it is desirable to connect the magnetic disk apparatus to a host controller not having such a function, and to connect the above-mentioned magnetic disk apparatus to such a host controller, it is necessary to change the firmware of the host controller. Such a change of the firmware would entail a great deal of trouble and, further, as mentioned above, would go against the principle that this magnetic disk apparatus is able to be connected without change to the host controller to which it is to be connected. Further, since read errors are recorded as statistical information, they would be treated as breakdowns. Therefore, from this standpoint too, it would become necessary to change the firmware of the host controller not having the above function. As mentioned above, changing the firmware would place a burden on the host controller side and would reduce the range of application of the magnetic disk apparatus.
Another method is to read the servo information of the comparative track when the select signal from the host controller is off, perform the update on the offset for correction of positioning, and then return the magnetic head to the original cylinder.
In this method, the host controller has a function of turning the select signal off. Also, changes in the firmware would be necessary. In particular, when only one magnetic disk apparatus is connected, it is often continually accessed and the select signal is not turned off. This being the case, problems similar to the above occur, i.e., there is no chance for correction of the offset in this method.